Conventional field emission flat panel display devices are convenient for use in applications which require display devices having less bulk, weight and power consumption than venerable cathode ray tube (CRT) display devices. As shown in FIG. 1, a conventional field emission display device 10 includes a baseplate 12 having a plurality of field-induced electron emitters 14 carried by a supporting substrate 16. The emitters 14 are disposed within respective apertures in an insulating layer 18 deposited on the surface of the supporting substrate 16. Also, a conductive layer forming an extraction grid 20 is deposited on the insulating layer 18 peripherally about the respective apertures of the emitters 14.
The conventional field emission display device 10 shown in FIG. 1 also includes a faceplate 22 having a transparent viewing layer 24 separated from the baseplate 12 by spacers (not shown) between the faceplate 22 and the baseplate 12. An anode 26 such as an indium tin oxide layer is deposited on a surface of the viewing layer 24 facing the baseplate 12. Also, localized portions of a luminescent layer 28 are deposited on the anode 26. The luminescent layer 28 typically comprises a phosphorescent material, such as a cathodophosphorescent material, which emits light when bombarded by electrons. A black matrix 30 is deposited on the anode 26 between the localized portions of the luminescent layer 28 to improve the contrast of the field emission display device 10 by absorbing ambient light.
In operation, a conductive voltage V.sub.c such as 40 volts applied to the extraction grid 20 and a source voltage V.sub.s such as 0 volts applied to the emitters 14 creates an intense electric field around the emitters 14. This electric field causes an electron emission to occur from each of the emitters 14 in accordance with the well-known Fowler-Nordheim equation. An anode voltage V.sub.a such as 1,000 volts applied to the anode 26 draws these electron emissions toward the faceplate 22. Some of these electron emissions impact on the localized portions of the luminescent layer 28 and cause the luminescent layer 28 to emit light. In this manner, the field emission display device 10 provides a display. Although the field emission display device 10 is shown in FIG. 1 having only two emitters 14 associated with each localized portion of the luminescent layer 28 for ease of understanding, those with skill in the field of this invention will understand that hundreds of emitters 14 may be associated with each localized portion of the luminescent layer 28 in order to average out individual differences in the electron emissions from different emitters 14.
In a conventional field emission display device configured as a monochrome display, each localized portion of the luminescent layer of the display device comprises one pixel of the monochrome display. Also, in a conventional field emission display device configured as a color display, each localized portion of the luminescent layer comprises a green, red or blue sub-pixel of the color display, and a green, a red and a blue sub-pixel together comprise one pixel of the color display. As a result, each pixel in a monochrome display and each sub-pixel in a color display is uniquely associated with one of the localized portions of the luminescent layer and hence is uniquely associated with a set of emitters.
If the electron emission from an emitter associated with a first localized portion of the luminescent layer of a conventional field emission display device also impacts on a second localized portion of the luminescent layer, then it causes both localized portions to emit light. As a result, a first pixel or sub-pixel uniquely associated with the first localized portion correctly turns on, and a second pixel or sub-pixel uniquely associated with the second localized portion incorrectly turns on. In a color display this can cause, for example, a purple light to be emitted from a blue sub-pixel and a red sub-pixel together when only a red light from the red sub-pixel was desired. This is obviously problematic because it provides a poor display.
This problem can be referred to as bleedover, and it can occur because the electron emission from each emitter in a conventional field emission display device tends to spread out from the baseplate of the display device. If the electron emission is allowed to spread out too far, it will impact on more than one localized portion of the luminescent layer of the display device. The likelihood that bleedover will occur is exacerbated by any misalignment between each localized portion of the luminescent layer and its associated set of emitters.
In conventional field emission display devices, bleedover is alleviated in three ways. First, the anode voltage V.sub.a applied to the anode of the conventional display device is a relatively high voltage such as 1,000 volts so the electron emissions from the emitters of the display device are rapidly accelerated toward the anode. As a result, the electron emissions have less time to spread out. Second, the gap between the baseplate and the faceplate of the conventional display device is relatively small, again giving the electron emissions less time to spread out. Third, the localized portions of the luminescent layer of the conventional display device are spaced relatively far from one another because of the relatively low display resolution provided by the conventional field emission display device. As a result, the electron emissions impact on the correct localized portion of the luminescent layer before they have a chance to impact on an incorrect localized portion.
However, as display designers attempt to increase the display resolution of the conventional field emission display device to provide a superior display, they necessarily crowd the localized portions of the luminescent layer of the display device closer together. As a result, bleedover begins to occur.
One solution to this problem might seem to be to decrease the distance between the faceplate and the baseplate of the conventional field emission display device. If this distance is decreased, the electron emissions from the emitters of the display device have less time to spread out and cause bleedover. However, it has been found that this is an impractical solution because the anode voltage V.sub.a applied to the anode of the display device needs to be as much as 1,000 volts or more in practice in order to adequately accelerate the electron emissions toward the anode. If the distance between the faceplate and the baseplate is decreased, arcing begins to occur between the faceplate and the baseplate because of this relatively high voltage. If, instead, the anode voltage V.sub.a is increased in order to accelerate the electron emissions toward the anode more rapidly and thereby prevent bleedover, arcing also begins to occur between the faceplate and the baseplate. Thus, there seems to be no practical way to both increase the display resolution of the conventional field emission display device and successfully prevent bleedover.
Therefore, there is a need in the art for a high display resolution field emission display device which successfully prevents bleedover.